Method of surface-treating zinc,aluminum and their alloys



United States Patent US. Cl. 148-62 14 Claims ABSTRACT OF THE DISCLOSUREA method is provided for treating the surfaces of zinc, aluminum, andtheir alloys to obtain a product having excellent corrosion resistance,paint adhesion properties and appearance. In accordance with the processan article having a surface of zinc, aluminum or an alloy thereof istreated with an aqueous solution of chromic acid and colloidal silica.

This invention relates to methods of treating the surfaces of zinc,aluminum, and their alloys and more particularly to surface-treatingmethods for preventing the formation so-called white rust on the zincsurfaced articles such as galvanized iron or steel sheets during storagein highly humid areas and shipment.

An object of the present invention is to provide a novel and improvedmethod for the treatment of an article having a surface of a metalselected from the group consisting of zinc, aluminum and their alloys toprotect the same from white rust or other corrosion by a simpletreatment without baking at a high temperature after applying a treatingsolution of the present invention to the said articles or withoutrinsing after drying them.

Another object of the present invention is to provide an improved methodfor the utilization of chromic acid or related chrominum compounds toprotect zinc-surfaced and aluminum-surfaced articles against white rust"or other corrosion.

A further object of the present invention is to provide a zinc-surfacedarticle which has no such surface coloring as is seen in anyconventional treating method, is very excellent in the adherence ofpaints and has a high commodity value.

Of the metallic coating available for the protection of iron and steel,zinc is the most desirable, because of (a) the excellent resistance ofzinc to atmospheric corrosion, (b) the anodic relationship of zinc toiron which provides sacrificial protection to exposed base metal and (c)the availability and low cost of zinc metal.

Specifically an iron or steel sheet plated with zinc is well known as agalvanized iron or steel sheet. The hot dipped galvanized process is oneof the cheapest methods of anticorroding iron or steel.

A zinc product has excellent resistance to atmospheric corrosion, but ina humid place, white rust will be produced on the surface of such zincproduct and not only the metallic luster on the surface will be lost butalso the corrosion resistance for the metal of the base will be reduced.Such white rust will impair the appearance and will reduce the commodityvalue. Specifically in such highly warm and humid land as Japan, whiterust or other corrosion will occur on the surface of such product duringthe transportation and will be also a cause of the return of theproduct.

On the other hand, aluminum coating or aluminized ferrous metal has beenalso recently developed and is applied as a corrosion resistant coatingfor iron and steel. But the same corrosion problem as in the case ofzinc has occurred in aluminum coating or aluminized ferrous metal.

3,506,499 Patented Apr. 14, 1970 For the prevention of formation ofwhite rust on zinc, there has been carried out a chromate treatingmethod wherein chromic anhydride, or various soluble chromate is mostlyused. For another method of the prevention of white rust, it is carriedout to absorb an oil layer .on the surface of zinc. But a zinc productsubjected to such treatment is so slippery as to be difficult anddangerous to handle.

The present invention relates to the above described chromate treatment.Such chromate treatment has been widely used for a method of antirustingnonferrous metals. The reaction mechanism in such treating method shallbe considered. In an anode part, zinc will be dissolved or oxidized. Ina cathode part, hexavalent chromium ions will be reduced and a chromatefilm mainly consisting of trivalent chromium will be formed on thesurface. In such case, if such anion accelerating the dissolution ofzinc as a sulfate ion is added, the reaction of both cathode and anodewill be accelerated and a relatively thick, corrosion resistant andcolored chromate film consisting of trivalent chromium will be formed onthe said surface.

In the surface-treatment of ordinary zinc-electroplating, the abovementioned method forming a relatively thick and colored chromate filmfrom a chromic acid bath containing sulfate ion is adopted and afavorable, corrosion resistant film having a good appearance is obtainedby rinsing after the chromate treatment and, as required, byneutralizing the treated surface with sodium carbonate or the like. Onthe other hand, in the case of a galvanized sheet, since the producingprocess is different, that is to say, a hot dipped galvanized processand a chromic acid treatment are carried out continuously, it isnecessary to carry out the surface-treatment as simply as possible.Therefore, a treating bath in which an excellent anticorrosive film isobtained only steps of chromic acid treatment, roll squeezing and dryingwithout baking at high temperature and water rinsing is most desired.

Typical of the surface treating methods for galvanized sheets whereinsaid steps are well utilized is a method wherein there is used a diluteaqueous solution of pure chromic acid. When this method is used, thesurface will not be altered from the original appearance and a corrosionresistant film to some extent will be obtained. On the other hand, asthe reaction between the zinc surface and a chromic acid will notproceed Well, though the appearance may be satisfactory, the corrosionresistance will not be sufiicient.

In view of the application to the simple steps of only the abovementioned dipping, roll-squeezing and drying, it is desirable to containno detrimental anion in the chromic acid bath but it is also necessaryto contain as few detrimental cations as possible. If such cations asalkali or alkaline earth metal ions or such anions as sulfate andchloride ions are present in the treating bath, they will have a badinfluence on the time variation of the surface of zinc, will impair theappearance and others with the lapse of time, will produce a white rustor other corrosion.

A chromic acid-water glass series has been recently proposed. However,Na ions are contained in water glass and will have a bad influence onthe time variation of the treated metal surface in the simple steps ofroll squeezing and drying.

Accordingly, the water rinsing after drying the film is an indispensableoperation. If the rinsing step is omitted, the corrosion resistance andpaint adhesion of the film will deteriorate. Such deterioration of thecorrosion resistance and paint adhesion of the film is considered to bedue to the Na ions in the water glass.

If a desired corrosion resistance is to be given to the treated film byomitting the rinsing after the drying by using the chromic acid-waterglass series treating solution, there may be considered the applicationof a soealled two solution type operation wherein the surface is dippedfirst in water glass and then in a chromic acid solution. However, insuch case, the process will be complicated and the loss of the treatingsolution will be apt to increase.

Here, if electro chemical consideration is applied to the reaction in aminute part of the zinc surface it will be possible to read oil? asuggestion whereby the difficulties in the case of applying the abovedescribed chromic acid-water glass series treating solution can besolved.

That is to say, in the reaction in the minute part of the zinc surface,in the cathode part, Cr will be reduced and, in the anode part, zincwill be dissolved. Therefore, if a substance which will be adsorbed inthe anode part and will give an anticorrosive film is added instead ofions to a pure chromic acid solution, it should be possible to improvethe corrosion resistance of the film and at the same time to reduce thetreating time.

As conforming to such condition is selected in the present invention achromic acid-colloidal silica series treating bath. Such uniformlydispersed colloidal silica will be negatively charged in the solution,will effectively compensate the defects of the chromate film produced inthe cathode part and will improve not only the corrosion resistance butalso the paint adhesion of the treated film and will reduce the dippingtreating time.

The treating solution according to the present invention is a solutionprepared by adding a colloidal silica substance to an aqueous solutionof chromic anhydride, or various soluble chromate. For the colloidalsilica substance is used a solution prepared by removing sodium in a slution of NaSiO (water glass) with an ion exchange resin or suchcommercial colloidal silica as, for example, Ludox produced by E. I. duPont de Nemours & Co., Inc., Wilmington, Del. Further, there can be useda silica acid colloid obtained by dialysis, electro dialysis,dissolution of elemental silicon, etc.

Anyhow, SiO must be uniformly dispersed in the form of a colloid in thesolution. Further, in the method wherein a part of hexavalent chromiumis reduced so that no detrimental oxidation product may be produced inthe treating solution, in case trivalent chromium is contained, it willbe more effective. For such method, there is used a method wherein suchreducing agent as saccharose is used or it is electrolytically reducedwith a carbon electrode.

The proper concentration of hexavalent chromium ions in the treatingsolution is 0.5 to 10 g./l. as metallic chromium or preferably 1 to 8g./l. Specifically, with the concentration of 1.2 to g./l., theappearance will be favorable and at the same time the corrosionresistance will be sufiicient. In the above, if the concentration is sohigh as to be, for example, more than g./l., the surface will be coloredand will be stained with the nonuniform deposition of chromic acid orvarious soluble chromate and further the amount of chromic acid orvarious soluble chromate taken out and deposited on the treated materialwill be so large that such high concentration will be eco nomicallydisadvantageous. Further, with a concentration of less than 0.5 g./l.,no favorable corrosion resistance will be shown.

The prior concentration of the colloidal silica is 1 to 70 g./l. as SiOThereby, not only the corrosion resistance will be improved but also thetreating solution will be uniformly deposited and a favorable film willbe obtained. In the above, with the addition of more than 70 g./l.,there will be a tendency of coloring and the corrosion resistance willreduce. With less than 1 g./l., the above mentioned effect will not comeout favorably. The range of addition most effective to give corrosionresistance is about 5 to 50 g./l. Specifically, in the case of additionof 20 g./l., the paint adhesion will be remarkable.

According to present treating method, a zinc or aluminum product isdegreased, is rinsed so that the surface may be clean and is then dippedin the treating bath at the room temperature for several seconds toabout 30 seconds, the excess of the treating solution deposited on theproduct being treated is removed with rubber rolls or any other wipingmechanism and the product is dried.

If the present treating solution is used, after the product is dried,there will be no need of rinsing it to improve the paint adhesion.

If the method of the present invention is adopted, even though norinsing is carried out after coating and drying, the anticorrosivenessand paint adhesion of the film will be very excellent.

Further, even if a two-solution method is applied to the presenttreating method and the material is treated first with only a colloidalsilica sol and then with chromic acid, there will be shown an effect notso different from the result of a one-solution treatment. As a result,in the treating method of the present invention, there is no need ofspecifically carrying out the two-step drying process in the case ofapplying the two-solution method, the treating process can be simplifiedand the drying time can be reduced very much.

The treating solution may be deposited on the surface of the material tobe treated not only by dipping but also by spraying or brush-coating theexcess treating solution may be removed and the material may be dried.

Examples of the present invention are given in the following.

EXAMPLE 1 Chromic anhydride5 g./l. Colloidal silica-20 g./l.Water--Rest.

Solution temperature25 C. Dipping time5 seconds.

EXAMPLE 2 Chromic anhydride-5 g./l. Saccharose1 g./l.

Colloidal silica (as SiO )20 g./l. Water'Rest.

Solution temperature-25 C. Dipping time5 seconds.

This treating solution was prepared by making chromic anhydride andsaccharose react well with each other by warming and then cooling them.

When a galvanized iron sheet treated with the above mentioned treatingsolution was subjected to a salt water spray test, no white rust wasseen to be produced at all in hours of salt water spraying.

For comparison, when the sheet treated with pure chromic acid to whichno colloidal silica had been added was subjected to the salt water spraytest, white rust was seen to be produced already in 10 to 12 hours and25 to 30% white rust was produced in 24 hours.

When the paint adhesion of the above mentioned materials was tested witha melamine alkyd series by a Frichsen cup test, the material treatedwith the chromic acid bath containing the colloidal silica sol was veryexcellent in the paint adhesion and showed satisfactory results.

Further, the amount of deposition of chromium was 0.2 mg./dm. in each ofExamples 1 and 2 and was also about 0.2 mg./dm. in case no colloidalsilica was added.

When the above mentioned treating bath was applied to aluminum, the sameexcellent corrosion resistance and paint adhesion as in zinc were shown.

What is claimed is:

1. A method of surface-treating metals consisting essentially of thesteps of coating the surface of a metal selected from the groupconsisting of zinc, aluminum, and their alloys with a mixed solution ofchromic acid containing 0.5 to 10 g./l. as metallic chromium and acolloidal silica containing 1 to 70 g./l. of SiO and drying the appliedsolution to form thin film on the said surfaces.

2. A method of surface-treating metals consisting essentially of thesteps of coating the surface of a metal selected from the groupconsisting of zinc, aluminum, and their alloys with a mixed solution ofchromic acid containing 1 to 8 g./l. as metallic chromium and acolloidal silica containing 5 to 50 g./l. of SiO and drying the appliedsolution to form a thin film on the said surfaces.

3. A method of surface-treating metals consisting essentially of thesteps of coating the surface of a metal selected from the groupconsisting of zinc, aluminum, and their alloys with a mixed solution ofchromic acid containing 1.2 to 5 g./l. as metallic chromium and acolloidal silica containing 5 to 50 g./l. of SiO and drying the appliedsolution to form a thin film on the said surfaces.

4. A method of surface-treating metals consisting essentially of thesteps of coating the surface of a metal selected from a group consistingof zinc, aluminum and their alloys first with a colloidal silicasolution containing 1 to 70 g./l. of SiO and then with chromic acidsolution containing 0.5 to g./1. as metallic chromium and drying theapplied solution to form a thin film on the said surfaces.

5. The method according to claim 1 wherein the coating is by dipping,spraying or brush-painting.

6. The method according to claim 1 wherein the coating comprises dippingsaid metal in the treating solution at the room temperature forsubstantially 1 to 30 seconds.

7. The method according to claim 1 wherein the surface of the metal is asurface of zinc, aluminum or its alloy with which is coated or platedthe surface of iron or steel.

8. A method of surface-treating metals consisting essentially of thesteps of coating the surface of a metal selected from the groupconsisting of zinc, aluminum and their alloys with a mixed solution ofchromic acid containing 0.5 to 10 g./l. as metallic chromium and acolloidal silica, and drying the applied solution to form a thin film onthe said surfaces.

9. A metal having a dry coating in intimate contact with a surfacethereof, the metal being a member selected from the group consisting ofzinc, aluminum and their alloys. the dry coating being in the form of athin film from an aqueous solution consisting essentially of water,chromic acid and from 1 to grams per liter of colloidal silica, theconcentration of hexavalent chromium ions in the aqueous solution beingfrom 0.5 to 10 grams per liter.

10. A coated metal according to claim 9 wherein the metal is zinc.

11. A coated metal according to claim 9 wherein the metal is aluminum.

12. A method comprising (a) coating a surface of a metal selected fromthe group consisting of zinc, aluminum and their alloys with a solutionconsisting essentially of water, chromic acid and from 1 to 70 grams perliter of colloidal silica, the concentration of hexavalent chromium ionsin the solution being from 0.5 to 10 grams per liter, and (b) drying thecoated surface to form a thin film thereon.

' 13. A method according to claim 12 wherein the metal is zinc.

14. A method according to claim 12 wherein the metal is aluminum.

References Cited UNITED STATES PATENTS 2,030,601 2/1936 McDonald148-6.16 2,680,081 6/1954 Probert et al 117135.1 2,989,418 6/1961Harbaugh 117135.1 X 3,013,897 12/1961 Cupery et al. 117135.1 X 3,133,8295/1964 Cupery et al. 117-1351 3,150,015 9/1964 Boyer et al. 148-6.22,850,415 9/1958 Harrison 148-6.2 X

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

